Abstract Systemic lupus erythematosus (SLE) is a painful, chronic autoimmune disease estimated to affect up to 150/100,000 people with an increased prevalence in young women. It results from disruption in normal immune tolerance mechanisms leading to activation of autoreactive T cells, expansion of autoreactive B cells, circulating autoantibodies and immune complex deposition. Current treatments consist primarily of high dose corticosteroids and immunosuppressive drugs that help to manage symptoms but fail to address the underlying cause and are associated with adverse side effects. Therefore, novel immunotherapeutic interventions are needed. Type I IFNs and the plasmacytoid dendritic cells (pDCs) that secrete them have emerged as key players in the pathogenesis of SLE. Immune cells from most SLE patients are imprinted with a type I IFN gene signature. Therefore, blockade of type I IFNs, their receptors and pDCs are being actively pursued as immunotherapies for SLE and other autoimmune diseases imprinted with IFN signatures. Despite the important immunological functions of pDCs, relatively little is understood about their molecular ?wiring? that controls IFN production, which presents a barrier to developing novel pDC-targeted immunotherapies. Herein, we provide evidence that the serine-threonine kinase, Tpl2 (also known as MAP3K8 or COT), is essential for TLR-induced type I IFN production by pDCs in vivo. The objective of this application is to understand how Tpl2 is uniquely required by pDCs for nucleic acid- induced IFN production and to determine whether Tpl2 expression in pDCs influences SLE pathogenesis. This will be examined in two Aims. Aim 1 will combine ex vivo biochemical analysis of primary murine pDCs with unbiased phosphoproteomics approaches to delineate the biochemical mechanisms by which Tpl2 promotes type I IFN production in pDCs. Aim 2 will use an innovative mixed bone marrow chimera approach to determine the contribution of Tpl2 expression within pDCs to SLE development a murine model. The expected outcome of the proposed studies is a better understanding of the molecular mechanism(s) governing pDC nucleic acid sensing and IFN production. This information will facilitate novel immunotherapeutic approaches to modulate IFNs and/or pDCs for treating SLE and possibly other human interferonopathies.